Polymer dispersions are normally produced by the process of emulsion polymerization in which water-insoluble monomers are polymerized in aqueous phase in the presence of emulsification or dispersion aids. The process may be used for the production of polymer dispersions from virtually any water-insoluble or only partly water-soluble monomers such as, for example, acrylic or methacrylic acid esters, vinyl esters, styrene, butadiene, chloroprene, acrylonitrile or vinyl chloride.
In general, the emulsifiers used are surface-active substances which control the course of the polymerization and which stabilize the product formed. Stabilizing means preventing individual polymer particles in the dispersions from coagulating to form relatively large agglomerates. The process of emulsion polymerization has been known for decades and is described in sufficient detail in standard works (see, for example, Rompps. Chemie Lexikon, 9th Edition, (1990), Thieme Verlag, Vol. 2, page 1159 and the literature cited therein).
Emulsion polymerization is used for the production of polymers which are used both as dispersions, for example in binders for paints or adhesives, and in bulk as a source for plastics or elastomers.
In the production of plastics or elastomers in bulk, the polymer dispersions are normally dried to remove the water present and to enable the plastic or the elastomer to be recovered in pure form. In many cases, the plastic or the elastomer is recovered by spray drying or by coagulation and subsequent drying. The necessary destruction of the dispersions in this process means that the polymer dispersions should not be particularly stable. Accordingly, it is preferred to use emulsifiers which, although enabling the dispersions to be produced without difficulty, do not unnecessarily complicate their subsequent destruction. Particular advantages are afforded by emulsifiers which can be specifically chemically modified in such a way that they completely lose their surface-active and stabilizing properties. The generally unwanted effect on the end product (for example sensitivity to water or lack of heat resistance) of the emulsifiers remaining therein can be avoided in this way.
This process is used in particular in the production of elastomers where the emulsifiers used are fatty soaps or resin soaps which can be converted into non-surface-active compounds by pH reduction. The latex produced and stabilized with soaps can be completely or partly coagulated by pH reduction, the fatty acids remaining in the elastomers as desired additives with no adverse effect as emulsifiers.
However, by comparison with other standard emulsifiers, such as alkyl sulfates or sulfonates, the soaps are poorer emulsifiers, i.e. although their use allows a polymerization to be carried out, a considerable percentage of coagulated polymer actually occurs during preparation of the emulsion or removal of the residual monomers as a result of shear forces, boiling processes or other mechanical influences. The coagulated fractions contaminate the polymerization reactor with each batch so that it has to be continually cleaned. Since the cleaning of the reactor reduces its capacity, attempts have been made to reduce the formation of coagulate by using secondary stabilizers (co-stabilizers).